Principle of operation (four-terminal resistivity measurement)
The soil resitivity measurement works on a similar principle to the other measurements which use stakes:
a current is injected around an outer loop and a voltage measured, shown in Figure 8. In this case,
however, the measurement made by the instrument requires further conversion using a formula to derive
the volumetric soil resistivity from the resistance value display.
Figure 8: schematic for 4-terminal resistivity measurement
For this test, the relative spacing and depth of the stakes is important. When configured as shown in
Figure 8, the soil resistivity can be calculated from the resistance value, R, displayed on the instrument as
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